1. Field of the Invention
The invention relates to methods of detecting and diagnosing lung cancers in mammals.
2. Brief Description of Related Art
Primary carcinoma of the lung is the leading cause of cancer death in the United States. Every year, more than 100,000 males and 50,000 females develop lung cancer, and most of them die within twelve months. There is a clinical need for a screening test which can detect lung cancer in its earliest stages because prompt treatment of localized disease improves the 5-year survival rate to 30% in males and 50% in females. However, most cases are not detected until local or metastatic growth causes symptoms, and prospective screening with frequent radiography and sputum cytology has not improved the survival rate in smoking males aged 45 years or older. Since early detection of lung cancer can potentially reduce mortality, researchers have investigated alternative diagnostic technologies such as breath testing.
The rationale of a breath test for lung cancer is based upon three observations: first, carcinogenesis is accompanied by increased production of oxygen free radicals (OFRs), second, OFRs degrade cell membranes by lipid peroxidation, evolving alkanes such as ethane and pentane, and third, these alkanes are volatile organic compounds (VOCs) which are excreted in the breath.
OFRs cause mutagenesis by oxidative damage to DNA; as a result, affected cells acquire malignant properties and tumor clones expand. The mutagenic effects of OFRs appear to be partially reversible; treatment with OFR scavengers significantly improved survival in metastatic gastric cancer as well as in a rat model of colon cancer induced by 1,2-dimethylhydrazine. Oxidative stress has also been associated with chemical toxicity, ischemia, inflammation and dietary deficiency of antioxidants. The final common pathway is the intracellular accumulation of OFRs which overcome cellular defense mechanisms and degrade cellular membranes by lipid peroxidation, resulting in chemical and anatomical disruption of the membranes which may progress to cell death.
Breath hydrocarbons, particularly alkanes such as pentane, are markers of oxidative stress mediated by OFRs. Increased breath pentane has been reported in breast cancer, acute myocardial infarction, heart transplant rejection, rheumatoid arthritis, and acute bronchial asthma. Previous studies have attempted to identify the VOCs in breath which might provide clinically useful markers of lung cancer. Gordon et al reported 28 VOCs which were present in the breath of more than 90% of patients with lung cancer; and Preti et al found increased o-toluidine;
Preti G, Labows J N, Kostelc J G and Aldinger S: Analysis of lung air from patients with bronchogenic carcinoma and controls using gas chromatography mass spectrometry. J. Chromatography 1988;432:1-11.
Gordon S M, Szidon J P, Krotoszynski B K, Gibbons R D and O'Neill H J: Volatile organic compounds in exhaled air from patients with lung cancer. Clin Chem 1985; 31:1278-82;
O'Neill H J, Gordon S M, O'Neill MH, Gibbons R D and Szidon J P: A computerized classification technique for screening for the presence of breath biomarkers in lung cancer. Clin Chem 1988; 34(8):1613-1618.
However, progress in breath testing for lung cancer has been impeded by the technical difficulty of detecting VOCs in breath. The majority are excreted in very low concentrations: nanomolar (10.sup.-9 mol/l) or picomolar (10.sup.-12 mol/l). Most existing laboratory instruments cannot detect VOCs in such low levels in breath unless the sample is concentrated prior to analysis. Researchers have circumvented this problem by constructing specialized instruments for the collection and concentration of breath samples. Phillips has recently described a method for the collection and analysis of breath VOC samples which can be employed in clinical settings. A portable microprocessor-controlled breath collection apparatus collects alveolar breath VOCs onto sorbent traps which are then analyzed by conventional gas chromatography and mass spectroscopy. This breath collection apparatus was utilized to collect samples from patients undergoing bronchoscopy and biopsy for suspected lung cancer, in order to correlate the VOCs in alveolar breath with the histopathologic screening tests.
Phillips M: Method for the collection and assay of volatile organic compounds in breath. Analytical Biochemistry 1997;247:272-278.